The present invention relates to a radiation computed tomography apparatus capable of acquiring volume data, such as a VCT (volume CT (computed tomography)) apparatus or a multi-row CT apparatus, and a tomographic image data generating method in the radiation computed tomography apparatus.
Particularly, the present invention relates to a radiation computed tomography apparatus and tomographic image data generating method for reducing the influence by scatter rays through correction.
Known X-ray CT apparatuses include, for example, one that acquires projection data for a subject by an X-ray detector having a plurality of X-ray detector elements arranged in a two-dimensional manner. The plurality of X-ray detector elements are positioned to have their width in a direction along a predefined axis with respect to the subject. Since X-ray detector element rows are formed over a certain width in the axis direction, the X-ray detector having X-ray detector elements arranged in a two-dimensional manner is generally referred to as a multi-row detector.
In the multi-row detector, a direction along the axis is sometimes referred to as a column direction, and a direction orthogonal to the column direction as a channel direction, for example.
In the X-ray CT apparatus comprising the multi-row detector, projection data of a cross section of the subject is acquired by emitting an X-ray fan beam, which has an extent in both the column and channel directions, from a predefined focal spot at a plurality of positions around the axis to the multi-row detector.
A tomographic image of the subject is produced by a reconstruction calculation based on the projection data.
Such an X-ray detector having a two-dimensional extent increases the probability that the detecting surface is struck also by X-rays other than those directly impinging upon a detecting surface from a focal spot, i.e., for example, by scatter X-rays (scatter rays) caused by X-rays having their direction of travel deflected due to collision with an object, such as bone, in the subject.
Especially, as an increase of the number of rows in the multi-row detector enlarges the width of the X-ray fan beam in the column direction, the fan beam is directed onto a wider area in the subject, resulting in generation of more scatter rays. As a result, the probability that scatter rays impinge upon the X-ray detector elements is increased accordingly.
The scatter rays contain projection information on the subject that the X-ray detector elements receiving the scatter rays should not detect. Therefore, the scatter rays cause generation of artifacts, and image quality of the tomographic image may be degraded.
To reduce such an influence by scatter rays, various techniques have been proposed. For example, Patent Document 1 discloses a CT apparatus for calculating a deconvolution function for reconstruction of a tomographic image from an arithmetically obtained elastic (coherent) scatter function of scatter rays, and reconstructing a tomographic image using the deconvolution function that corrects the influence by coherent scatter rays.                [Patent Document 1] Japanese Patent Application Laid Open No. H7-184886.        
In the CT apparatus described in Patent Document 1, however, the width of opening (aperture) for an X-ray beam in the column direction is not particularly taken into account. Thus, it cannot fully deal with the multi-row detector.
As explained above, conventional techniques do not fully correct and reduce the influence by scatter rays, which increases with increasing number of X-ray detector rows, so that there is a need to further improve image quality of tomographic images.